1. Field of the Invention
This invention generally relates to electric motors, and more specifically to a motor construction wherein the stator coils are arranged to maximize the flux coupled into the rotor space or air gap and, therefore, the rotor.
2. Description of Related Art
The operation of a motor relies upon effective coupling of magnetic flux or field developed in the stator poles into the rotor air gap or space and, therefore, the rotor. However, in most electrical machines there is considerable fringe or leakage flux which bypasses the rotor space, and, therefore, is not coupled to the rotor because magnetic flux takes the path of least resistance in the magnetic circuit, or the path of lowest magnetic reluctance. Since the opposing poles in a two pole motor are in the shape of circular cylindrical surfaces, to accommodate the cylindrical rotor, the extreme portions of the opposing poles, which are the closest to each other, offer the path of least resistance to the magnetic flux and a significant portion of that flux bridges such portions of the opposing poles and, thereby, bypasses the rotor. This fringe or leakage flux serves no useful purpose and prevents the motor from achieving optimum operational or performance characteristics.
In U.S. Pat. No. 4,482,832, issued on Nov. 13, 1984, a shaded pole motor is disclosed which uses a lamination design which improves the geometry of the inter-pole gap. According to the patentee, this is intended to improve the operating efficiency of the motor. This is achieved in the patent by providing a reluctance gap having a base region of minimal cross-sectional area. This design recognizes the problem created by fringe or leakage flux. Since fabrication of the laminations is substantially simplified by providing a single continuous lamination which forms both opposing poles, the reduction of the cross-sectional area to the minimum possible dimension increases the magnetic reluctance of the smaller sections of magnetic material, this forcing a greater amount of the flux through the rotor air gap or space. The situation resembles an electrical circuit of parallel resistors. The rotor space or gap between the poles represents a reluctance to the magnetic field which is substantially greater than the reluctance presented by the magnetic material forming the laminations. By decreasing the amount of metal bridging or connecting the poles, the reluctance to the magnetic field continues to increase thereby forcing a greater and greater amount of the flux through the air gap or rotor space.
The foregoing U.S. Pat. No. 4,482,832 discloses a very common type of fractional horsepower AC shaded pole electric motor construction. In this type of motor, there is provided a stator core which is basically U-shaped, including two side portions or arms and a connecting or cross portion provided with the cylindrical opening for the rotor. The end portions of the arms are bridged by a yoke which supports the coil. Application of an AC signal to the coil in such a motor causes a magnetic intensity in the magnetic circuit with the resulting flux splitting into two components as aforementioned. The exent to which the flux will pass through the rotor space will be a function of the thickness of the magnetic material connecting or bridging the opposing pole faces. However, even if the metal were theoretically entirely removed, so that there was true physical separation of the opposing poles, there would still be a significant amount of leakage or flux because the regions where the opposing poles are the closest are at these diametrically opposite sides of the rotor and this, again, presents the lowest reluctance path or path of least resistance for the flux.
There have been numerous proposals for positioning stator coils on motors. One example of such motor is disclosed in U.S. Pat. No. 2,253,524. This patent discloses a reversible motor including a primary stator coil positioned on the yoke as discussed in connection with U.S. Pat. No. 4,482,832. However, additional stator coils are provided on or in the region of the cross or bridging portion of the stator core and positioned on opposing sides of the rotor air gap or space. These coils are connected in different electrical configurations designed to permit easy reversal of the direction or rotation of the motor. In essence, the reference discloses the creation of multiple poles spaced from each other with respect to the axial or medial plane of the rotor to permit the enhancement of flux in different portions of the rotor. However, the motor disclosed in U.S. Pat. No. 2,253,524 suffers the same disadvantages above discussed since substantial amounts of flux field are bypassed or shunted around the motor and, therefore, serve no effective or useful function. Additional examples of reversible-type shaded pole motors are disclosed in the following U.S. Pat. Nos.: 2,027,846; 2,066,760; and 2,567,976.